Dimerization reactions photodimerization

Photodimerization reactions of some other simple alkenes and dienes follow/39-30 36-182 Although not a dimerization reaction, photochemical ring closures to yield cyclobutane derivatives are analogous and are included in this section 31-35 ... 

The efficient enantioselective dimerization reaction of 97 in the inclusion complex (100) with 12a suggests that two coumarin molecules are arranged in chiral positions in 100 and their [2 + 2] photodimerization occurs by keeping the chirality in 100. The chiral arrangement of 97 was detected by CD spectral measurement of the inclusion complex in Nujol mulls. The 1 1 complex of 97 with 12a and 13a showed CD spectra with a mirror-imaged relation (Fig. 17). After photoirradiation, the CD absorptions of the complex (100) at 225, 275, 300, and 330 nm disappeared, and the new CD absorption due to the inclusion complex (101) of 98 at 240 nm appeared. The photodimerization of 97 was also followed by measurement of infrared (IR) spectra as Nujol mulls. Upon photoirradiation, the nCO absorption of 97 in 100 at 1700 cm 1 decreased gradually and finally disappeared after 4 hr, and new vCO absorption due to 98 in 101 appeared at 1740 cm-1. 

The steric course of the photodimerization reactions of coumarin (37a) and thiocoumarin (37b) succeeded in being controlled almost perfectly by carrying out the reaction in inclusion complexes using various host compounds. Furthermore, enantioselective dimerization reactions of 37a and 37b were found to proceed through a single-crystal to single-crystal process. 

Photodimerization of 2-pyridone (46) in the presence of the 2,2/-biphenyldi-carboxylic acid host (45) also proceeded via a catalytic process. First, irradiation of the 1 2 inclusion complex of 46 and 45 in the solid state gave the trans-anti dimer (47) in 92 % yield [27], The mechanism of this stereoselective photoreaction was investigated through X-ray analysis of this complex. In the complex, two 46 molecules are arranged in ideal positions for yielding 47 by dimerization [27], Secondly, a catalytic dimerization reaction of 46 was carried out. Photoirradiation for 20 h of a 1 4 mixture of powdered 45 and 46 under occasional mixing in the solid state gave 47 in 81 % yield. These data clearly show that molecules of... 

Carbon attack of the carbonyl group of cyclopropyl ketones has also been achieved in dimerization reactions under McMurry and photochemical conditions. Conceivably, the reactions take place via radical anion intermediates. Photodimerization does not occur if structural features of the substrate facilitate intramolecular reactions such as ring opening, ring expansion, and ring closure. Radical... 

Dimerization.—The dihydropyrones (40a and b) do not photodimerize, whereas the photodimerization of (40c) shows an interesting solvent dependence.28 Thus in hexane the head-to-head and head-to-tail dimers are obtained in a 65 35 ratio, whereas in acetonitrile only the former is produced. In contrast, (40d) shows no solvent dependence in its dimerization reaction and yields only the... 

Both the [4+4] photodimerization reaction (4 — 6) and the isomerization to give Dewar-pyridone products (4 —> 5) tolerates substitution at every position (Table 1), but certain combinations of substituent and position are notable exceptions. The parent 2-pyridone 4a and N-methyl 4b are typical, yielding either 5 or 6 depending on the starting concentration. - A single additional allyl substituent, represented by l,n-dimethyl examples 4c-4f, all yield at least the [4+4] dimer. Even a /-butyl substituent at C4 (4g) which presents a possible steric impediment, is compatible with the dimerization reaction. Fusion of a cyclohexyl at C5/C6 (4k) or a cyclohexadienyl ring at C4/C5 (41) leads to acceptable yields of the photodimer 6. Chlorine substitution... 

The dimerization reaction takes place upon irradiation with light of a wavelength longer than 300 nm. It was demonstrated subsequently, however, on poly(vinyl cinnamate) that adducts dissociate again upon irradiation with light of 254 nm. Photodimerization and formation of cyclobutane groups is enhanced as a result of sensitized irradiation of ethyl and other cinnamate. 

Figure 4 General reaction scheme for the [2-1-2] photodimerization reactions of chalcone and dibenzylidene acetone. In each case, the dimerization reaction can potentially give rise to four possible dimers syn head-to-head (product 1), anti head-to-head (product 2), syn head-to-tail (product 4) and anti head-to-tail (product 3).

Cinnamoyl groups were introduced into the amino- and carboxyl-bearing blocks of the copolymers, because we initially intended to take advantage of the photodimerization reaction of the cinnamoyl units (Guo et al, 1996 Liu et al, 1999 Stewart and Liu, 2000) and thus stitch together the associated polymer chains. However, the amidization reaction was later found to be more efficient than the cinnamoyl dimerization reaction. Therefore, the cinnamoyl units performed no apparent function in this study. 

Connection of two cinnamate chromophores has frequently been used to attain high local concentrations of reactants, thereby circumventing the inefficiency of the above-mentioned bimolecular dimerization reactions. An extreme example of this approach is embodied in the cyclophane structures 3, in which two cinnamate moieties undergo photodimerization to give quantitatively the P-truxinate dimer with a quantum yield of Similarly, the bis-cinnamamide 4 furnished 48% of P-truxinimide upon... 

Many macrocyclic compounds have been prepared by the irradiation of oqto-dicinnamates and dicoumaryl derivatives (130, 130-16). > When the tether is a polyglycol unit, the photodimerization is sensitive to alkali metal ions, and crown-ether-Hke structures can be prepared. Intramolecular dimerization reactions can find preparative importance as, for instance, in the preparation of the P- and 8-truxinate dimers from the irradiation of propylene-1,3-dicinnamate and n-butylene-l,4-dicinnamate, respectively. The tether may also function as a chiral auxihary, such as in the irradiation of L-erythritol 1,4-dicinnamate or mannitol hexacinnamate. In the latter case, an excess of the (-l-)-8—truxinate isomer was observed, along with several other isomers. 

In the crystal of 1,4-dicinnamoylbenzene (1,4-DCB) (see Fig. 12), the distances between the intermolecular photoadductive carbons are 3.973 and 4.086 A for one cyclobutane ring, and 3.903 and 3.955 A for the other. The two topochemical pathways may occur competitively in a single crystal of 1,4-DCB at the initial stage of reaction. Then, both intramolecular photodimerization and intermolecular photopolymerization of the diolefinic mono-cyclobutane intermediate occur competitively to give tricyclic dimer 21,22,23,24-tetraphenyl-l,4,ll,14-tetraoxo-2(13),12(13-diethanol, [4.4] para-cyclophane or oligomers (Hasegawa et al., (1985). On photoirridation at room temperature the 1,4-DCB crystal gives >90% of the tricylic... 

Intermolecular photocycloadditions of alkenes can be carried out by photosensitization with mercury or directly with short-wavelength light.179 Relatively little preparative use has been made of this reaction for simple alkenes. Dienes can be photosensitized using benzophenone, butane-2,3-dione, and acetophenone.180 The photodimerization of derivatives of cinnamic acid was among the earliest photochemical reactions to be studied.181 Good yields of dimers are obtained when irradiation is carried out in the crystalline state. In solution, cis-trans isomerization is the dominant reaction. 

Derivatives of anthracene bearing substituents on the 1 or 2 position can be photodimerized with efficiencies comparable to that for the unsubstituted molecule. However, with substituents at the 9 meso) or 9, 10 dimeso) positions a very interesting photochemical problem results. Since dimerization occurs across the 9, 10 positions, substituents at these positions exert a first-order effect on the photochemical reaction. The mero-substituted anthracenes examined include the following as 19>... 

The stereospecificity of these reactions is surprising in light of the large energies absorbpd by these molecules. Indeed, the major photochemical product of these photolyses was the alternate olefin isomer (1-butene was also observed). These results indicate that free rotation about the photo-excited double bond does not occur in those molecules that dimerize. This suggests the participation of ground state complexes or excimers in the photodimerization. This view is supported by the observations that dilution of cw-2-butene with neopentane (1 1) decreased the yield of dimers and a 1 4 dilution almost completely suppressed dimerization. 

Cycloadditions, although relatively rare, are not unknown in heterocyclic systems. The first documented example was the photodimerization of pyrid-2-one a reinvestigation of this reaction has established that three other isomeric [ 4 + 4] dimers are formed in low yield in addition to the originally reported trans-anti dimer (264).215 Attempts to effect analogous cycloadditions in a series of l,T-polymethylenedipyrid-2-ones were unsuccessful, [ 2 + 2] and [ 4 + 2] additions being preferred.216 Thus,... 

The first photochemical reactions to be correlated with PMO theory were the dimerizations of anthracene, tetracene, pentacene, and acenaphthylene. 36> More detailed energy surfaces for the photodimerization reactions of butadiene have also been calculated. 30> In the category of simplified calculations lie studies of the regiospecificity of Diels-Alder reactions 37>, and reactivity in oxetane-forming reactions. 38,39) jn these... 

In this type of process an excited molecule adds to a second — identical — molecule in its ground state, usually with formation of a ring. These dimerizations are thus most commonly intermolecular reactions, but obviously the two reactive moieties can also be linked together, e.g. by an alkyl chain. Such intramolecular photodimerization reactions have been studied in detail422). 

Cyclic a,P-enones photodimerize via the Tt-state, as in (4.23)426) and (4.24)427). By chosing the appropriate substituents the reactions proceed regio- and stereoselectively. In contrast, pyrimidine bases dimerize from the first excited singlet state to afford head to tail products (4.25) 428). 

An asymmetric photosynthesis may be performed inside a crystal of -cinnamide grown in the presence of E-cinnamic acid and considered in terms of the analysis presented before on the reduction of crystal symmetry (Section IV-J). We envisage the reaction as follows The amide molecules are interlinked by NH O hydrogen bonds along the b axis to form a ribbon motif. Ribbons that are related to one another across a center of inversion are enantiomeric and are labeled / and d (or / and d ) (Figure 39). Molecules of -cinnamic acid will be occluded into the d ribbon preferentially from the +b side of the crystal and into the / ribbon from the — b side. It is well documented that E-cinnamide photodimerizes in the solid state to yield the centrosymmetric dimer tnixillamide. Such a reaction takes place between close-packed amide molecules of two enantiomeric ribbons, d and lord and / (95). It has also been established that solid solutions yield the mixed dimers (Ila) and (lib) (Figure 39) (96). Therefore, we expect preferential formation of the chiral dimer 11a at the + b end of the crystal and of the enantiomeric dimer lib at the —b end of the crystal. Preliminary experimental results are in accordance with this model (97). 

The second example is an intermolecular crystal-state reaction. Cross-conjugated 1,5-disubstituted 1,4-dien-3-ones in solution undergo both cis-trans photoisomerization and photodimerization, yielding complex mixtures of products, including die all-trans-substituted cyclobutane 2 in the case of 1,5-diphenyl-1,4-pentadien-3-one. In contrast, dienones such as 3a in whose crystals adjacent molecules lie parallel and strongly overlapped react in the solid to give 3b as the sole photoproduct. This isomerically pure tricyclic diketone results, formally, from an eight-center dimerization. It is not formed in the reaction in solution, and could be prepared by other methods only with considerable difficulty (4). 

We turn first to the (4 + 4) photodimerization of anthracenes, which has been most extensively studied in this context. In many anthracenes it has been possible to show that in the starting crystals defects are present at which the structure is appropriate for formation of the observed dimer in others it has been argued that the presence of such defects is very plausible. The weakness of this interpretation, at this stage, is that in no case has it yet proved possible to establish that the reaction indeed occurs at these defect sites. 

Photodimerization of cinnamic acids and its derivatives generally proceeds with high efficiency in the crystal (176), but very inefficiently in fluid phases (177). This low efficiency in the latter phases is apparently due to the rapid deactivation of excited monomers in such phases. However, in systems in which pairs of molecules are constrained so that potentially reactive double bonds are close to one another, the reaction may proceed in reasonable yield even in fluid and disordered states. The major practical application has been for production of photoresists, that is, insoluble photoformed polymers used for image-transfer systems (printed circuits, lithography, etc.) (178). Another application, of more interest here, is the use that has been made of mono- and dicinnamates for asymmetric synthesis (179), in studies of molecular association (180), and in the mapping of the geometry of complex molecules in fluid phases (181). In all of these it is tacitly assumed that there is quasi-topochemical control in other words, that the stereochemistry of the cyclobutane dimer is related to the prereaction geometry of the monomers in the same way as for the solid-state processes. 

The culmination of the studies on asymmetric photodimerization reactions in the solid state was the successful elaboration of chemical systems that are achiral but crystallize in chiral structures, and that yield, on irradiation, dimers, trimers, and higher oligomers in quantitative enantiomeric yield (175,258). 

Photodimerization of coumarin and its derivatives have been widely investigated and used.[37-45] It is known that under a UV irradiation with light wavelength longer than 300 nm, coumarin and its derivatives can undergo [2+2] dimerization to form four types of dimmers depending on the reaction conditions. Importantly, such formed photodimers can be... 

Toda et al. reported that the topotactic and enantioselective photodimerization of coumarin and thiocoumarin takes place in single crystals without significant molecular rearrangements [49]. Molecular motion needs to be called upon to explain the photochemically activated cycloaddition reaction of 2-benzyl-5-benzylidenecyclopentanone. The dimer molecules, once formed, move smoothly in the reactant crystal to form the product crystal [50]. Harris et al. investigated the reactivity of 10-hydroxy-10,9-boroxophenanthrene in the solid state and the mechanism of the solid-state reaction was characterized by both X-ray diffraction and thermal analysis [51]. It was demonstrated that the solution chemistry of 10-hydroxy-10,9-boroxophenanthrene is different from that in the solid state, where it undergoes dimerization and dehydration to form a monohydride derivative. 

It may be suspected that the genuinely topotactic (as secured by the molecular precision of the AFM [18]) photodimerization of 2-benzyl-5-benzyli-denecyclopentanone [118] might be a good candidate for a quantitative preparative photo dimerization to give the head-to-tail anti-[2+2] dimer. Early quantitative solid-state [2-1-2] photodimerizations (most of the published mechanistic interpretations of which can no longer be accepted) are listed in [110]. These deal with the anti dimerization of acenaphthylene-1,2-dicarboxylic anhydride, the head-to-head syn dimerization of acenaphthylene-1-carboxylic acid, the syn dimerization of 5,6-dichloroacenaphthylene, and the thermally reversible head-to-tail anti dimerization of seven ( )-2,6-di-f-butyl-4-(2-aryl-ethenyl)pyrylium-trifluoromethanesulfonates. All of these reactions proceed fully specific. On the other hand, quantitative photoconversions of a 1 1 mixed crystal of ethyl and propyl a-cyano-4-[2-(4-pyridyl)ethenyl]cinnamates gives mixtures of diesters with one (A>410 nm) or two cyclobutane rings (no cutoff filter). 

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